Acoustic signals, for example, beep sounds or voice, may be particularly useful for safety critical applications such as operation of a motor vehicle, e.g., a motor car. An acoustic signal, e.g., an audible signal, may be advantageous compared to, e.g., an optical signal in that the acoustic signal may be more likely to be perceived by the user, e.g., by the operator of a machine such as the driver of a motor vehicle or a person working in a control room of, e.g., a factory, power plant, port, airport, or train station. While an optical signal may be perceived only when the signal is within the field of view of the user, the perception of an audio signal may be substantially independent of the current field of view of the user.